Striving for perfection is demoralizing. Striving for excellence is motivating.

Harriet Braiker (1948-2004) Psychologist, consultant, author

The Disease to Please, ch. 2 (2001)

A parallel quote in the book is "Striving for perfection is a demoralizing and guaranteed formula for failure. Striving for excellence, on the other hand, is motivating because reaching it is attainable."

More often quoted as: "Striving for excellence motivates you; striving for perfection is demoralizing."

Full text.